Magnetostatic Interactions in Self-Assembled Co_<i>x</i>Ni_1–<i>x</i>Fe₂O₄/BiFeO₃ Multiferroic Nanocomposites

Self-assembled vertically aligned oxide nanocomposites consisting of magnetic pillars embedded in a ferroelectric matrix have been proposed for logic devices made from arrays of magnetostatically interacting pillars. To control the ratio between the nearest neighbor interaction field and the switching field of the pillars, the pillar composition Co_<i>x</i>Ni_1–<i>x</i>Fe₂O₄ was varied over the range 0 ≤ <i>x</i> ≤ 1, which alters the magnetoelastic and magnetocrystalline anisotropy and the saturation magnetization. Nanocomposites were templated into square arrays of pillars in which the formation of a “checkerboard” ground state after ac-demagnetization indicated dominant magnetostatic interactions. The effect of switching field distribution in disrupting the antiparallel nearest neighbor configuration was analyzed using an Ising model and compared with experimental results

Thickness-Dependent Double-Epitaxial Growth in Strained SrTi_0.7Co_0.3O_3−δ Films

Perovskite-structured SrTi_0.7Co_0.3O_3−δ (STCo) films of varying thicknesses were grown on SrTiO₃(001) substrates using pulsed laser deposition. Thin films grow with a cube-on-cube epitaxy, but for films exceeding a critical thickness of about 120 nm, a double-epitaxial microstructure was observed, in which (110)-oriented crystals nucleated within the (001)-oriented STCo matrix, both orientations being epitaxial with the substrate. The crystal structure, strain state, and magnetic properties are described as a function of film thickness. Both the magnetic moment and the coercivity show maxima at the critical thickness. The formation of a double-epitaxial microstructure provides a mechanism for strain relief in epitaxially mismatched films

Ordered Nanoscale Archimedean Tilings of a Templated 3‑Miktoarm Star Terpolymer

The directed self-assembly of 3-miktoarm star terpolymer chains (polyisoprene-<i>arm</i>-polystyrene-<i>arm</i>-polyferrocenylethylmethylsilane (3 μ-ISF)) into 2D Archimedean tilings is described. A morphological change from (4.8²) to (6³) tiling is reported in the 3 μ-ISF thin film blended with PS homopolymer when a greater swelling of PI is achieved during the solvent annealing process. Highly oriented (4.8²) tilings were produced by templating the self-assembled three colored structures in blended thin films. The use of (4.8²) and (6³) tilings as nanolithographic masks to transfer square and triangular hole arrays into the substrate is also demonstrated

Optimizing Topographical Templates for Directed Self-Assembly of Block Copolymers via Inverse Design Simulations

An inverse design algorithm has been developed that predicts the necessary topographical template needed to direct the self-assembly of a diblock copolymer to produce a given complex target structure. The approach is optimized by varying the number of topographical posts, post size, and block copolymer volume fraction to yield a template solution that generates the target structure in a reproducible manner. The inverse algorithm is implemented computationally to predict post arrangements that will template two different target structures and the predicted templates are tested experimentally with a polydimethylsiloxane-<i>b</i>-polystyrene block copolymer. Simulated and experimental results show overall very good agreement despite the complexity of the patterns. The templates determined from the model can be used in developing simpler design rules for block copolymer directed self-assembly

The Spatial Resolution Limit for an Individual Domain Wall in Magnetic Nanowires

Magnetic nanowires are the foundation of several promising nonvolatile computing devices, most notably magnetic racetrack memory and domain wall logic. Here, we determine the analog information capacity in these technologies, analyzing a magnetic nanowire containing a single domain wall. Although wires can be deliberately patterned with notches to define discrete positions for domain walls, the line edge roughness of the wire can also trap domain walls at dimensions below the resolution of the fabrication process, determining the fundamental resolution limit for the placement of a domain wall. Using a fractal model for the edge roughness, we show theoretically and experimentally that the analog information capacity for wires is limited by the self-affine statistics of the wire edge roughness, a relevant result for domain wall devices scaled to regimes where edge roughness dominates the energy landscape in which the walls move

Morphology Control in Block Copolymer Films Using Mixed Solvent Vapors

Solvent vapor annealing of block copolymer thin films can produce a range of morphologies different from the equilibrium bulk morphology. By systematically varying the flow rate of two different solvent vapors (toluene and <i>n</i>-heptane) and an inert gas, phase maps showing the morphology <i>versus</i> vapor pressure of the solvents were constructed for 45 kg/mol polystyrene-<i>block</i>-polydimethylsiloxane diblock copolymer films of different thicknesses. The final morphology was correlated with the swelling of the block copolymer and homopolymer films and the solvent vapor annealing conditions. Self-consistent field theory is used to model the effects of solvent swelling. These results provide a framework for predicting the range of morphologies available under different solvent vapor conditions, which is important in lithographic applications where precise control of morphology and critical dimensions are essential

Aligned Sub-10-nm Block Copolymer Patterns Templated by Post Arrays

Self-assembly of block copolymer films can generate useful periodic nanopatterns, but the self-assembly needs to be templated to impose long-range order and to control pattern registration with other substrate features. We demonstrate here the fabrication of aligned sub-10-nm line width patterns with a controlled orientation by using lithographically formed post arrays as templates for a 16 kg/mol poly(styrene-block-dimethylsiloxane) (PS-<i>b</i>-PDMS) diblock copolymer. The in-plane orientation of the block copolymer cylinders was controlled by varying the spacing and geometry of the posts, and the results were modeled using 3D self-consistent field theory. This work illustrates how arrays of narrow lines with specific in-plane orientation can be produced, and how the post height and diameter affect the self-assembly

Templated Self-Assembly of a PS-<i>Branch</i>-PDMS Bottlebrush Copolymer

The self-assembly of block copolymers (BCPs) with novel architectures offers tremendous opportunities in nanoscale patterning and fabrication. Here, the thin film morphology, annealing kinetics, and topographical templating of an unconventional Janus-type “PS-<i>branch</i>-PDMS” bottlebrush copolymer (BBCP) are described. In the Janus-type BBCP, each segment of the bottlebrush backbone connects two immiscible side chain blocks. Thin films of a Janus-type BBCP with <i>M</i>_n = 609 kg/mol exhibited 22 nm period cylindrical microdomains with long-range order under solvent vapor annealing, and the effects of as-cast film thickness, solvent vapor pressure, and composition of the binary mixture of solvent vapors are described. The dynamic self-assembly process was characterized using in situ grazing-incidence X-ray scattering. Templated self-assembly of the BBCP within lithographically patterned substrates was demonstrated, showing distinct pattern orientation and dimensions that differ from conventional BCPs. Self-consistent field theory is used to elucidate details of the templated self-assembly behavior within confinement

Interfacial Energy-Controlled Top Coats for Gyroid/Cylinder Phase Transitions of Polystyrene-<i>block</i>-polydimethylsiloxane Block Copolymer Thin Films

Block copolymers (BCPs) with a high Flory–Huggins interaction parameter (χ) can form well-defined sub-10 nm periodic structures and can be used as a template for fabrication of various functional nanostructures. However, the large difference of surface energy between the blocks commonly found in high-χ BCPs makes it challenging to stabilize a useful gyroid morphology in thin film form. Here, we used an interfacial-energy-tailored top-coat on a blended film of a polystyrene-<i>block</i>-polydimethylsiloxane (PS-<i>b</i>-PDMS) BCP and a low-molecular-weight PDMS homopolymer with a hydrophilic end functional group. The top coat consisted of a random mixture of 40% hydrolyzed poly­(vinyl acetate)-<i>random</i>-poly­(vinly alcohol) (PVA-<i>r</i>-PVAc, PVA40) and PVAc homopolymer. At the optimized top-coat composition, gyroid nanostructures with sub-10 nm strut width were achieved down to ∼125 nm film thickness, which is only 3 times the lattice parameter of the gyroid structure. This is in marked contrast with a mixed morphology of gyroid and cylinders obtained for other compositions of the top coat. Self-consistent field theoretic simulations were used to understand the effect of the interfacial energy between the top coat and BCP/homopolymer blends on the phase transition behavior of the BCP/homopolymer films

Magnetic Phase Formation in Self-Assembled Epitaxial BiFeO₃–MgO and BiFeO₃–MgAl₂O₄ Nanocomposite Films Grown by Combinatorial Pulsed Laser Deposition

Self-assembled epitaxial BiFeO₃–MgO and BiFeO₃–MgAl₂O₄ nanocomposite thin films were grown on SrTiO₃ substrates by pulsed laser deposition. A two-phase columnar structure was observed for BiFeO₃–MgO codeposition within a small window of growth parameters, in which the pillars consisted of a magnetic spinel phase (Mg,Fe)₃O₄ within a BiFeO₃ matrix, similar to the growth of BiFeO₃–MgFe₂O₄ nanocomposites reported elsewhere. Further, growth of a nanocomposite with BiFeO₃–(CoFe₂O₄/MgO/MgFe₂O₄), in which the minority phase was grown from three different targets, gave spinel pillars with a uniform (Mg,Fe,Co)₃O₄ composition due to interdiffusion during growth, with a bifurcated shape from the merger of neighboring pillars. BiFeO₃–MgAl₂O₄ did not form a well-defined vertical nanocomposite in spite of having lower lattice mismatch, but instead formed a two-phase film with in which the spinel phase contained Fe. These results illustrate the redistribution of Fe between the oxide phases during oxide codeposition to form a ferrimagnetic phase from antiferromagnetic or nonmagnetic targets